This invention relates to systems for generating Fourier transforms of multifrequency signals, and more particularly to systems and methods using wave propagation and diffraction to partition a frequency band while retaining the full information content in the subdivided band or bands.
The mathematical generalization for the information-bearing signal of finite duration is referred to as the Fourier transform or integral. As signal processing techniques have advanced and applications have expanded there has arisen an increased need for systems and methods for effecting Fourier transformation of multifrequency input signal bands to enable meaningful information to be extracted from different frequency components within the band.
When processing broadband signals to detect the existence of one or more signal frequencies within the band or to partition the wider band into narrower subbands, it is common to employ frequency scanning techniques. Scanning techniques are sequential in nature and therefore not suitable for situations in which a number of transmitted signal frequencies must be continuously collected or monitored. Filter banks of conventional design can be complex and expensive, especially when designed to provide precise partitioning of a wide frequency band. Spectrum analyzers based upon distributed frequency-transform techniques, sometimes using surface acoustic wave devices, are used for some specific applications.
For other contexts in which small size and concurrent signal partitioning are required, there have been developed what are known as optical spectrum analyzers. These analyzers use an acousto-optic modulator through which a collimated light beam (e.g. laser beam) is transmitted. A transducer attached to one side of the modulator generates ultrasonic waves corresponding to and responsive to the signals within the frequency band under investigation. The presence of energy at given frequencies in the spectrum causes deviation of the beam through frequency dependent angles, so that one or more of an array of distributed light sensors is illuminated concurrently to identify the active frequency bands. However, such systems are relatively complex because of the presence of the laser, and also present inherent nonlinearities because of the acousto-optic interaction. Equally importantly, they can preserve the phase information of the incoming signal only through the use of complicated optical heterodyning techniques. They are also strictly unidirectional in character. Consequently, they are not of general applicability.
Workers in the art generally recognize a parallel between optical waves and acoustic waves, as illustrated by a number of articles in which various techniques for the steering or sensing of a beam are used for purposes of frequency selectivity. For example, in Electronics Letters for 31 May 1973, Volume 9, No. 11, at pp. 246 and 247, subject matter of this type was disclosed by P. Hartemann in an article entitled "Frequency-Selective Scanning Of Acoustic Surface Wave". The principle of using a multi-source transducer that generates a collimated beam and launches surface acoustic waves at a variable direction in a surface acoustic wave substrate toward one of a number of output transducers is described in relation to an experimental system. The concept of using a collimated beam and directing it at various angles toward receiving transducers presents significant problems. A very long path length between input and output transducers is needed to provide frequency selectivity while avoiding interference between adjacent transducers. Also, as illustrated by the frequency responses of FIG. 2b and the measurements represented in Table 1 of the article, the insertion losses and side lobes are high, and the number of frequencies that may be detected are consequently low for useful operative values.
An article entitled "Frequency-Controlled Beam Steering Of Surface Acoustic Waves Using A Stepped Transducer Array", by R. M. De La Rue et al, published in the Electronics Letters, 9, 15, pp. 326-327, July 26, 1973, describes the construction and operation of a multi-element transducer array in which the elements are arranged linearly along an anisotropic substrate. This construction demonstrates that a surface acoustic wave beam may be steered in one direction or another in correspondence with frequency deviations from the center frequency. It is proposed that this may be used to switch between two or more separate receiver transducers or to provide frequency-band separation.
A related system is described in an article entitled "Scanning Of Surface Acoustic Wave Phased Array" by Tsai et al in Proceedings of the IEEE, June 1974, pp. 863 to 864. The article also proposes the use of interdigital transducers placed side-by-side perpendicular to the nominal acoustic propagation path and discloses changing the direction of scan by varying the phase of the drive to each transducer, although frequency scanning is also mentioned. The article also proposes the use of such a transducer array in a different fashion, in which signals received by the antenna elements of a phased-array antenna would be applied on a 1:1 basis to the transducers, so as to be capable of detecting applied signals simultaneously. A related technique is described by the same authors in an article entitled "Surface Acoustic Wave Array Transducers And Their Applications" in the Symposium On Optical And Acoustical Micro Electronics, pp. 583 to 597 (1974). In this article the plane array is supplemented by a stepped array of elements and the frequency scanning approach is discussed in greater detail. It is proposed, very generally at page 595, to provide the function of an acousto-optic spectrum analyzer, by arranging a number of output transducers along a circumference at a far-field location of the acoustic beam. However, a practical system for this purpose is not discussed, and a long path length would again be needed to achieve clear separation of the collimated waves, unless employing an acoustic lens as briefly mentioned. Further consideration shows that the proposed approach encounters severe problems if it is desired to have a high level of frequency discrimination, high signal-to-noise ratio and realistically low side lobes. Furthermore, it is now understood that consideration must be given to more subtle coactions between the input array and the output array as well as the anisotropy of the propagating medium.
Subsequent work by P. Hartemann and P. Cauvard is presented in an article entitled "Wavefront Synthesis And Reconstruction Using Acoustic Surface Waves", published in the 1977 Ultrasonic Symposium Proceedings, IEEE Cat. No. 77CH1265-1 SU. This system is based upon the use of a circular array of surface acoustic wave point sources, disposed on an isotropic medium, and fed through a tapped surface acoustic wave delay line for frequency steering. This substrate limits the frequencies that may be used to approximately 50 MHz, and the arrangement is inefficient because the large angle of divergence from each transducer wastes energy among many diffracted orders. Essentially the same system was again later described with further detail by the same authors, with others, in an article entitled "Ultrasound Beam Scanning Driven By Surface-Acoustic-Waves" published in the 1978 Ultrasonic Symposium Proceedings, IEEE Cat. No. 78CH1344-1 SU, pp. 269-272.
Such prior art systems essentially demonstrate the feasibility of operation of different parts of a wide band signal partitioning system, but they do not directly confront many, often conflicting, requirements imposed by advanced systems applications. In order to use higher center frequencies, and to cover wider bandwidths, problems unrecognized and unaddressed by the prior art must be overcome. As frequency increases, the propagation losses in a piezoelectric substrate increase, and it becomes more difficult to obtain a large fractional bandwidth and a low insertion loss. In addition, one must consider the practical limitations of lithography and other reproduction processes that can be used in making economically acceptable devices and systems. In this regime of high frequency, wide bandwidth applications, frequency selectivity and system sensitivity become of significance, particularly where it is desired to identify relatively brief and low signal amplitude components of unknown frequency within a wide bandwidth.
The band partitioning functions which have been discussed exemplify some of the problems involved in Fourier transform processors for multifrequency signals. In addition, the particular mode of the transformation should not limit system capability by destroying phase information or requiring complex processing for information retrieval. From the practical standpoint the system must be physically realizable using reliable manufacturing techniques and must operate substantially uniformly throughout a wide bandwith.